A substrate which has an Si layer on an insulating layer is known as an SOI (Silicon On Insulator) substrate. As SOI substrate manufacturing methods, SIMOX (Separation by IMplantation of OXygen), bonding, and the like are known.
SIMOX is a technique for implanting oxygen ions in an Si substrate at a predetermined depth and then annealing the substrate at a high temperature to form a buried silicon oxide layer.
Bonding is a method of bonding the first substrate which has a single-crystal Si layer on a separation layer and an insulating layer on the Si layer to the second substrate to form a bonded substrate stack and then dividing the bonded substrate stack at the separation layer. A method of forming a porous Si layer as the separation layer and epitaxially growing the single-crystal Si layer on the porous Si layer is known as ELTRAN (ELTRAN is a registered trademark). A method of using an ion-implanted layer as the separation layer to divide the bonded substrate stack by annealing is known as Smart Cut (Smart Cut is a registered trademark).
Typically, ELTRAN (registered trademark) selectively etches porous silicon left on the surface of an SOI layer after dividing the bonded substrate stack. In some cases, the surface of the SOI layer is rough after the selective etching. Japanese Patent Laid-Open No. 5-217821 discloses a technique for making the surface of the SOI layer extremely smooth by performing hydrogen annealing for an SOI substrate.
Recently, SOI layers have rapidly been thinning. A technique related to thinning of SOI layers is disclosed in Japanese Patent Laid-Open No. 2001-168308. Japanese Patent Laid-Open No. 2001-168308 discloses that in the manufacture of a thin silicon film, the thin silicon film arranged on an insulating surface is subjected to wet cleaning and is reduced in thickness to 100 nm or less. Japanese Patent Laid-Open No. 2001-168308 also discloses that an SC-1 cleaning solution (a solution mixture of NH4OH, H2O2, and H2O) is preferable as a cleaning solution, and an organic alkaline solution or a mixture of hydrofluoric acid and nitric acid can also be used. In addition, Japanese Patent Laid-Open No. 2001-168308 discloses a technique for reducing the thickness of a thin silicon film to 100 nm or less.
At present, the SOI layer in an SOI substrate has a sufficiently acceptable level of film thickness uniformity. The film thickness uniformity between substrates, however, is expected to have a tolerance of about ±1 nm. In a method using ion implantation, variations between substrates in depths where ions are implanted can cause variations in thicknesses of SOI layers. In a method using epitaxial growth, variations in thicknesses of single-crystal Si layers to be formed by epitaxial growth can lead to variations in thicknesses of SOI layers.
To reduce variations in the SOI layer between substrates in thicknesses, a polishing step or etching step can be adopted. Since polishing is a machine work, it is difficult for the present to stop polishing so as to maintain a high precision of ±1 nm or less with respect to a target film thickness. The rate of silicon etching using an SC-1 cleaning solution, an organic alkaline solution, a mixture of hydrofluoric acid and nitric acid is sensitive to the temperature and concentration of the chemical solution. To control the thickness of a thin silicon film at high precision (e.g., ±1 nm), these conditions must be controlled at extremely high precision.
In thinning of an SOI layer, defects such as a through hole (pinhole) and the like may increase with a decrease in thickness of the SOI layer. More specifically, the manufacturing process of an SOI substrate can adopt, e.g., a division step in which a mechanical stress can be applied to a layer to serve as an SOI layer and/or a polishing step. A stress which may be generated by these steps is more likely to cause defects with a decrease in thickness of the layer to serve as the SOI layer.
Under the circumstances, to meet demands for a thinner SOI layer, there is available a method of forming an SOI layer with a thickness large enough to prevent defects and then thinning the SOI layer to a desired thickness in the final step. To implement thinning, polishing, sacrificial oxidation (thermal oxidation and etching of a thermal oxide film), etching using an SC-1 cleaning solution, or the like may be preferable. However, in polishing, the SOI layer is mechanically damaged to cause defects. In sacrificial oxidation, for example, the thermal oxidation rate depends on the type and doping amount of an impurity doped in an Si layer, and thus variations are likely to occur in oxidation amount (the thickness of an oxidized portion). Also, the doping amount may change by thermal diffusion. In both of sacrificial oxidation and etching using the SC-1 cleaning solution, the surface roughness of the SOI layer may degrade.
Also, support for various types needs to be considered. In many cases, the specification value for the thickness of an SOI layer is determined by an application-specific demand from a device manufacturer as the user. For this reason, to meet a large number of demands, an SOI manufacturing process flexible toward a required film thickness must be established. For example, to manufacture ten types of SOI substrates with thicknesses of 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 nm by a SIMOX process using ion implantation, it is necessary to establish process conditions including ten types of ion implantation conditions corresponding to the thicknesses and manage them.
A conventional etching process is controlled mainly by the process time. If process conditions vary due to a change in concentration of a chemical solution or the like, it is difficult to precisely control the etching amount or film thickness.
Consequently, in repeating the etching process and film thickness measurement until a desired film thickness is obtained, the following operation is repeated. More specifically, a substrate to be processed is removed from the holder of a cleaning unit, the film thickness is measured by a film thickness measurement unit, and the substrate to be processed is attached again to the holder of the cleaning unit. This makes the work inefficient.